This invention relates to a cargo handling system that manipulates loads and pallets within the cargo bay of a wide-bodied aircraft. In more general terms however, the teachings and principles of the invention may be applied to any conveyance system where it is desired to propel a load from one point to another.
"Cargo Power Drive Units," or "cargo PDUs," are part of an integrated system for conveying aircraft loads within an aircraft's interior. Luggage and other cargo are typically loaded into large metal bins that may be easily moved into the aircraft, to quickly prepare the aircraft for departure, and easily unloaded from the aircraft upon arrival. These metal bins fit into the belly of the aircraft, and are stacked within the aircraft along its length for flight. Large cargo loaders are used to load these bins through a cargo bay door of the aircraft, so that the bins may subsequently be moved along the length of the aircraft and stowed.
A cargo PDU is an electro/mechanical roller device that propels these loads within the aircraft, both from side-to-side and along the length of the aircraft. There are other applications and configurations for these PDUs and their support elements within the cargo bay. For example, their arrangement and configuration may depend upon the type of aircraft and whether the aircraft is configured to transport cargo, or just passengers. The aircraft cargo bays may typically contain many different types of loads and pallets besides the metal bins just mentioned.
These loads and pallets are normally supported by a system of freely rotating bearings, as shown in FIGS. 1 and 2, that support the loads and pallets and enable them to be easily pushed within the aircraft to their intended position. Cargo PDUs were developed to enable these loads and pallets to be so positioned without requiring extensive manpower. Thus, with cargo PDUs, it is possible for a single operator to electronically control the manipulation of these loads and pallets within the aircraft's cargo bay.
Cargo PDUs are most feasible in large aircraft where many loads must be accommodated, and therefore typically find their home as part of an integrated conveyance system aboard a wide-bodied aircraft. Groups of these cargo PDUs are intermittently placed along the length of the aircraft in one or more rows so that multiple containers may be moved and positioned within the aircraft's belly.
Some of the first cargo PDUs consisted of tires having two motors and a gearing system, and were normally hidden below the cargo deck. The tires were actuated by the first motor to raise up above the cargo deck for driving the load. The second motor then caused rotation of the tire so that the loads could be mechanically positioned within the aircraft. However, these early systems had several faults: Besides requiring a large amount of room beneath the floor of the cargo bay for their installation and support, these systems would lock in the event of a power failure, preventing ready manual unloading of the aircraft's cargo bay. That is, if power was unavailable, these tires were locked in position above the cargo deck, and thus prevented the pallets and loads in the cargo bay from being manually pushed over the freely rotatable support bearings that would enable their ready movement if the cargo PDUs were not present. With other similar cargo PDUs, the raising and lowering of these rollers was accomplished by a mechanical system that was separate from the motor used to rotate the wheel. These faults were not relieved by implementation of these mechanical systems, because they too required extensive manpower.
In the late 1960s and early 1970s, a breakthrough was developed. A company called Western Gear developed a "self-lifting cargo PDU." That is, Western Gear developed a power drive unit that used the same motor both to lift a roller from beneath the cargo bay floor into a position to where it would contact a load or pallet riding on the bearing support system, and also to rotate the roller so that the load could be propelled. Since the same motor performed both functions, the rotation of the roller was mechanically connected to the mechanical elements that caused the roller to lift through the cargo bay floor. If the power was unavailable and the motor was locked, the roller was not locked in an elevated position, but rather, would retract under the influence of gravity. Thus, if a power failure occurred, these new self-lifting cargo power drive units would retract and allow manual removal of loads and pallets in the aircraft's cargo bay.
This early basic innovation has enjoyed enormous success. Its principles have been used in nearly every type of wide-bodied aircraft to provide an efficient, mechanical system for loading and unloading aircraft cargo bays. In addition, these self-lifting cargo PDUs were shallow in profile and thus could easily be mounted in an aircraft cargo bay without requiring much space or weight.
Naturally, many have strived to make superior cargo power drive units that would operate even more efficiently. The two primary design considerations that faced makers of cargo power drive units were (1) the weight of each unit, since this could significantly affect an airline's fuel costs over time, and (2) long life and reliability.
An aircraft may have several different banks or sets of cargo PDUs. For example, in a typical configuration, an aircraft will have two cargo bays, each divided into two sections, front and back, each section having cargo PDUs organized into left and right tracks. The aircraft may also have an additional track along the width of the aircraft, positioned at the door, to allow loads to be moved into and out of the aircraft through the cargo bay doors and also transferred between the left and right cargo PDU tracks. Some newer aircraft may also feature an additional "kink" section, or sloped section in the aircraft's tail, which may have additional left and right cargo PDU tracks. The aircraft 25, shown in FIG. 1, is one illustrative configuration and features one cargo bay, having a front section and a rear "kink" section 11, which has a different angle of inclination from the rest of the cargo bay.
Each set of PDUs, consisting of a plurality of PDUs arranged along a path of conveyance, are driven in common. That is, an operator manipulating the PDU controls, which consist essentially of an on/off switch and a joy stick, will cause all of the PDUs in a given set to lift and rotate in the same direction at the same time. This enables the operator to simultaneously move more than one pallet or load within the aircraft. In the system shown in FIGS. 1-2, there are thus five sets of cargo PDUs: Front section left and right track sets 13, 15, rear (kink) section left and right track sets 17, 19, and a lateral set 21 positioned at the cargo bay door 23 that connects left and right tracks at the front/rear section junction.
For example, as there are typically numerous loads to be stowed within the aircraft, several loads are manipulated simultaneously by one energized set of cargo PDUs. However, a first load or pallet reaching the end of the aircraft will be blocked, and unable to move further while the entire set of energized cargo PDUs continues to rotate in elevated condition and propel the remainder of the loads or pallets along the length of the aircraft. The rollers underneath a jammed load thereby abrasively "scrub" the bottom of the load, causing damage to the pallet and wear to the PDU wheel or roller. The rubber material of the PDU, when so damaged, causes rubber dust to be generated, which further increases required maintenance to the aircraft cargo bay. This "scrubbing" is perhaps the most significant obstacle to PDU roller life.
Solutions have been proposed to solve the proble of cargo PDU roller wear. Most of these have consisted, however, of attempts to construct the cargo PDU roller or wheel out of a rubber material that is not easily worn by the scrubbing activity. While these solutions have helped, they have not eliminated the problem.
In an optimal environment, it is desired that whenever a load or pallet hits an end stop, cargo PDUs beneath the load or pallet should be stalled such that the roller no longer rotates in trying to propel the load or pallet. Of course, once a PDU is stalled, any scrubbing is stopped because, although the roller remains in contact with the load's bottom, the roller stops rotating. This "stalling" may be achieved by virtue of the fact that cargo pDUs are driven by electric motors that utilize a maximum of three-phase two-hundred volt voltage (at 400 hertz). Thus, when the roller is prevented from rotating, it is desired that the resistance to the motor causes the motor to cease operation until reenergized.
In current configurations, however, the maximum voltage is always supplied, because aircraft bays may vary in their orientation to gravity. Most airports are sloped at a slight angle away from passenger terminals to allow proper drainage. Thus, a normally level aircraft cargo bay may have gravitational tilt (longitudinal) or roll (lateral), or both, during normal loading and unloading. Since the force of roller rotation is proportional to the inverse square of the cargo PDU supply voltage, the maximum of two-hundred volts is typically applied to all energized cargo PDUs, to enable their continued operation in all environments, including worst case situations involving maximum tilt.
The force required by a cargo PDU to propel a load may vary greatly depending upon the angle of inclination. For example, a PDU may typically require a force of 35 pounds to move a load on a level surface. When the load must be propelled against a 3 degree inclination, this force may be increased to in excess of 200 pounds. To solve this problem, the maximum of two-hundred volts is supplied to all energized PDUs. That is, PDUs are always operated under the worst case assumptions and supplied with two-hundred volts, and thereby always supply over 200 pounds of roller drive force. This greater force makes it very difficult for jammed loads to stall underlying PDUs and thus is a primary cause of the "scrubbing" problem. PDUs do not, in fact, always require the maximum supply voltage, because cargo bay tilt or roll is usually minimal. Even when an aircraft bay is significantly tilted, only a few of the load conveyance paths face an uphill slope.
Thus, there is a need for an invention that provides for less scrubbing and cargo bay maintenance necessitated by roller wear, and that facilitates stalling when a load contacts an end stop or is otherwise prevented from moving. The current invention satisfies this need, and provides a new generation of cargo systems that present a significant advance and improved efficiency as yet unachieved by the systems described above.